The use of computer systems has grown so pervasive that the power consumed by computer systems has become a concern for computer system designers and consumers. To reduce the cost of providing power to operate computer systems and the corresponding consumption of energy resources, the goal of designing a "green PC" that consumes less power has been pursued by several manufacturers. Manufacturers of mobile or "portable" computer systems that operate using rechargeable batteries as power supplies have also attempted to reduce power consumption so that the mobile computer system may be used for extended periods of time without recharging the batteries.
To reduce power consumption and to extend battery life, much of the integrated circuitry used as components of computer systems is being designed to operate at low voltage levels. For example, the circuitry and components used in portable computers are being designed to operate exclusively at voltage levels such as five volts and 3.3 volts. This reduces power consumption and allows more components to be placed closer to one another in the circuitry.
Unfortunately, the movement towards reducing the power consumption of computer systems may conflict with the desire to provide after-market upgrades and add-on devices for portable computer systems. One type of device that may be used to increase the versatility of a portable computer system is the flash electrically erasable programmable read only memory ("flash EEPROM"). Flash EEPROMs are nonvolatile memory devices that can be programmed and erased by the user, and flash EEPROMS may be used, for example, as BIOS ROMs or as part of a plug-in memory card. Flash EEPROMs typically require higher voltages for programming and erasing data than can be provided directly by the reduced voltage power supplies of green PCs and portable computers.
One solution for allowing flash EEPROMs to be used in reduced voltage computer system designs is to provide charge pump circuits external to the flash EEPROMs for boosting the supply voltage levels of the computer system to the higher voltage levels required by the flash EEPROM. A difficulty with this solution is that the use of separate charge pump circuits requires printed circuit board space that may be at a premium in a portable computer system.
An alternative solution is to design flash EEPROMs that include charge pump circuits for internally generating the higher voltage levels required by the flash EEPROM. One difficulty with this solution is that charge pumps internal to the flash EEPROM require semiconductor die space, which may require an increase in the semiconductor die size for the flash EEPROM. Another difficulty is that internal charge pump circuits may not be able to provide sufficient current to program and erase the memory cell array as quickly as external charge pumps, and operation of the flash EEPROM may be slowed.